Heat-sensitive type flow rate detecting element and holder therefor

ABSTRACT

A flow rate detecting element measuring the flow rates of various fluids, particularly the intake air of an internal combustion engine. The flow rate detecting element has a thin film layer including a support film and a protective film on one surface of a flat substrate, a heating resistance section and a comparative resistance section thermosensitive resistor having patterns and located between the support film and the protective film. The flat substrate has a recess which penetrates the flat substrate in the thickness direction thereof and facing the heating resistance section and the comparative resistance section. A fluid flow passage communicates with the recess which faces the comparative resistance section for fluid flow into the recess. Flow rate or velocity of a fluid can be measured accurately using the heating resistance section according to the fluid temperature reported by the comparative resistance section.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flow rate detecting element used in athermosensitive flow rate sensor which measures the flow rate of afluid. The present invention also relates to an element holder whichaccommodates the flow rate detecting element when the flow ratedetecting element is put into practical use.

The flow rate detecting element of the present invention can be used formeasuring the flow rate of various fluids such as gases and/or liquids,preferably gases, and particularly air and/or a mixture of gases. Theflow rate detecting element of the present invention can be preferablyused for measuring the flow rate of, in particular, the air intake intoan internal combustion engine.

2. Description of the Related Art

The applicant of this patent application has filed an application forpatent (Japanese Patent Kokai Publication No. 11-23338) related to athermosensitive flow rate detecting element based on an operatingprinciple similar to that of the present invention. FIG. 9(a) shows aschematic plan view of a key portion in a basic embodiment of theinvention of the above Japanese Patent Kokai Publication, and FIG. 9(b)shows a schematic sectional view taken along lines IXB—IXB in FIG. 9(a).

In FIG. 9, reference numeral 31 denotes a flat substrate made of asilicon semiconductor, 32 denotes an insulating support film made ofsilicon nitride, 34, 35, 36 and 37 denote thermosensitive resistors madeof, for example, platinum, where 34 is a heating resistance section, 35and 36 are temperature detecting resistance sections and 37 is acomparative resistance section, and 33 denotes an insulating protectivefilm made of silicon nitride.

As shown in FIG. 9(b), the flat substrate 31 has a recess 38 in theright-hand portion thereof, formed by removing a part of the substratein a predetermined size and shape by etching or other method. The flatsubstrate 31 also has a notch 39 in the left-hand portion thereof,formed by removing a part of the substrate in a predetermined size andshape by etching or other method, so that the notch 39 having a crosssection of substantially triangular shape with the base thereof lying atthe lower surface of the substrate 31 does not reach the upper surfaceof the substrate 31. The terms “lower surface” and “upper surface” areused herein in correspondence to the lower and upper positions in thedrawings which show the longitudinal sectional view of the flow ratedetecting element of the present invention, for the convenience ofdescription.

On the upper surface of the flat substrate 31, the support film 32 andthe protective film 33 are sequentially laminated. Provided between thesupport film 32 and the protective film 33 are the heating resistancesection 34, the temperature detecting resistance sections 35, 36, andthe comparative resistance section 37 being formed in predeterminedpatterns as shown in FIG. 9(a). A portion enclosed by two-dot and a dashline in FIG. 9(a), which includes the heating resistance section 34 andthe temperature detecting resistance sections 35, 36 that are providedon both sides of the heating resistance section 34, constitutes aheating resistance region section 40. The heating resistance region 40consists of a thin film comprising the support film 32 and theprotective film 33 placed one on another, and constitutes a so-calleddiaphragm structure with the recess 38 formed on the bottom side so asnot to contact the flat substrate 31.

Formed in the flat substrate 31 on the lower surface side of thecomparative resistance section 37 is the notch 39 which opens only onthe lower surface of the flat substrate 31. A portion of the flatsubstrate 31 remains on the side which is in contact with the supportfilm 32.

In use condition of the flow rate detecting element having theconstitution described above, the resistance sections 34, 35, 36 and 37are connected to circuits not shown. When a fluid, for example air,flows in the direction indicated by arrow 50, the comparative resistancesection 37 contacts with the flowing air via the protective film 33 tosense the temperature of the air. The temperature of the heatingresistance section 34 is set to remain higher than the temperature beingmeasured at the comparative resistance section 37 by a predeterminedmargin. For the application to an internal combustion engine ofautomobile, for example, temperature of the heating resistance section34 is controlled to maintain a level 200 degree centigrade higher thanthe temperature being measured at the comparative resistance section 37.

Heat generated by the heating resistance section 34 is transmitted tothe temperature detecting resistance sections 35, 36 via the supportfilm 32 and the protective film 33. Since the temperature detectingresistance section 35 and the temperature detecting resistance section36 are disposed at positions symmetrical with respect to the heatingresistance section 34 which is located at the center as shown in FIG. 9,there is no difference in the resistance between the temperaturedetecting resistance section 35 and the temperature detecting resistancesection 36 when there is no fluid flow. Also since the comparativeresistance section 37 is located at a predetermined distance from theheating resistance section 34, heat generated by the heating resistancesection 34 is substantially not transmitted to the comparativeresistance section 37, so that the temperature of the comparativeresistance section 37 is nearly equal to the temperature of thesurrounding fluid, for example air.

When a fluid, for example air, flows in the direction indicated by thearrow 50 over the flow rate detecting element having such a constitutionas described above, since the temperature of the heating resistancesection 34 is set to a level generally higher than the fluid temperatureto be measured, the temperature detecting resistance section 35 locatedin the upstream is cooled by the fluid to a lower its temperature. Thetemperature detecting resistance section 36 located in the downstream,on the other hand, receives the heat generated by the heating resistancesection 34 and conveyed by the fluid, and therefore shows either a lessdrop in the temperature or a rise in the temperature. As a result, whenthe fluid flows in the direction indicated by the arrow 50, temperatureof the temperature detecting resistance section 35 located in theupstream becomes lower than that of the temperature detecting resistancesection 36 located in the downstream, while the difference in theresistance between the two temperature detecting resistance sections 35and 36 becomes larger as the flow rate or the velocity of the fluidincreases. Thus the flow rate or the velocity of the fluid can bemeasured by sensing the difference in the resistance between thetemperature detecting resistance section 35 and the temperaturedetecting resistance section 36.

When the fluid flows in a direction opposite to the arrow 50, since thetemperature of the temperature detecting resistance section 36 locatedin the upstream becomes lower than that of the temperature detectingresistance section 35 located in the downstream, contrary to the casedescribed above, direction of the fluid flow can also be determined.

The thermosensitive flow rate detecting element as described above isaccommodated in an element holder when it is practically used, in orderto avoid the turbulence of the fluid flow and to achieve an effectivecontact of the fluid with the heating resistance section or thecomparative resistance section. The applicant of this patent applicationhas also filed an application for patent (Japanese Patent KokaiPublication No. 10-293052) on the element holder.

The flow rate detecting element described above measures flow rate bymeans of the heat transmission phenomenon of the fluid. Therefore, anaccurate monitoring of the fluid temperature is required in order tomeasure the flow rate accurately. That is, when the fluid temperaturevaries, the comparative resistance section 37 provided on the substrateis required to detect the change without delay. When measuring an amountof the flow rate of air intake into an internal combustion engine, forexample, there may be such occasions while running as the intake airtemperature indicates a sudden change at, for example, the entry andexit of a tunnel. To have the internal combustion engine operate withthe best performance even in such cases, the air temperature change mustbe detected quickly and accurately. Consequently, the flow ratedetecting element is required to have good response characteristics withrespect to the intake air temperature.

Silicon has a relatively large thermal capacity. As a result, in casethe substrate 31 made of silicon is provided under the comparativeresistance section 37, the comparative resistance section 37 has greaterapparent thermal capacity which results in a limitation to theimprovement of the response characteristics of the flow rate detectingelement with respect to the fluid temperature changes.

SUMMARY OF THE INVENTION

With the background described above, such an attempt has been made thatintroduces the diaphragm structure for the comparative resistancesection 37 similarly to the heating resistance region 40. However, thecomparative resistance section 37 is required to have even bettertemperature response characteristic which means that the time requiredto detect the fluid temperature change is desired to be as short aspossible. Thus the comparative resistance section 37 is required to haveeven better temperature response characteristic even when the diaphragmstructure is employed.

First object of the invention of the present application is to provide aflow rate detecting element having a comparative resistance section madein such a structure that has an improved temperature responsecharacteristic, by placing emphasis particularly on the temperatureresponse characteristic of the comparative resistance section in thethermosensitive flow rate detecting element described above.

Second object of the invention of the present application is to providean element holder which accommodates the thermosensitive flow ratedetecting element having the comparative resistance section made in sucha structure that has an improved temperature response characteristic,when put in practical use.

The flow rate detecting element according to the first aspect of thepresent invention has a thin film layer comprising a support film and aprotective film which are made of insulating material and are formed onone surface of a flat substrate, where a heating resistance section anda comparative resistance section are provided by disposingthermosensitive resistor in predetermined patterns between the supportfilm and the protective film of the thin film layer, and the flatsubstrate has a recess which penetrates the flat substrate in thedirection of thickness thereof in at least a part thereof that faces theheating resistance section and the comparative resistance section, sothat the thermosensitive flow rate detecting element measures the flowrate or velocity of a fluid by means of the heating resistance sectionaccording to the report of fluid temperature sensed by the comparativeresistance section, while a fluid flow passage is provided whichcommunicates with the recess that faces the comparative resistancesection thereby to cause the fluid to flow to the recess.

The flow rate detecting element constituted as described above makes itpossible to bring the fluid into sufficient contact also with the lowersurface of the comparative resistance section having the diaphragmstructure, by the fluid flow passage communicating with the recess thatfaces the comparative resistance section and flowing the fluid smoothlythrough the fluid flow passage into the recess. Thus since thecomparative resistance section of the flow rate detecting element canmake contact with the fluid on both the upper surface and the lowersurface thereof and the fluid which contacts the lower surface alsoflows quickly through the fluid flow passage, the fluid temperature canbe sensed sensitively. As a result, the flow rate detecting element canmake quick response to the temperature change even when the fluidtemperature changes suddenly.

The flow rate detecting element according to the second aspect of thepresent application is a variation of the flow rate detecting element ofthe first aspect, wherein at least two fluid flow passages are provided.

In the flow rate detecting element constituted as described above, thefluid can be caused to flow more smoothly into the recess by arrangingat least one of the fluid flow passages at an fluid inlet-side of therecess and at least one of the fluid flow passages at an fluidoutlet-side of the recess. Therefore, the comparative resistance sectionof the flow rate detecting element can sense the fluid temperatureaccurately, and the flow rate detecting element can make quick responseto the temperature change even when the fluid temperature changessuddenly.

The flow rate detecting element according to the third aspect of thepresent application is a variation of the flow rate detecting element ofthe first aspect, wherein at least one fluid flow passage is provided inthe upstream of the comparative resistance section in the main flowdirection of the fluid to be measured.

In the flow rate detecting element constituted as described above, thefluid is made easier to flow through the fluid flow passage into therecess so that the fluid can flow more smoothly into the recess byproviding at least one fluid flow passage in the upstream of thecomparative resistance section in the main flow direction of the fluidto be measured. Therefore, the comparative resistance section of theflow rate detecting element can sense the fluid temperature sensitively,and the flow rate detecting element can make quick response to thetemperature change even when the fluid temperature changes suddenly.

The flow rate detecting element according to the fourth aspect of thepresent application is a variation of the flow rate detecting element ofthe first aspect, wherein at least one fluid flow passage is provided inthe downstream of the comparative resistance section in the main flowdirection of the fluid to be measured.

In the flow rate detecting element constituted as described above, thefluid is made easier to flow through the fluid flow passage into therecess so that the fluid can flow more smoothly into the recess, byproviding at least one fluid flow passage in the downstream of thecomparative resistance section in the main flow direction of the fluidto be measured. Therefore, the comparative resistance section of theflow rate detecting element can sense the fluid temperature sensitively,and the flow rate detecting element can make quick response to thetemperature change even when the fluid temperature changes suddenly.

The flow rate detecting element according to the fifth aspect of thepresent invention is a variation of the flow rate detecting element ofthe first aspect, wherein the comparative resistance section and theheating resistance section are disposed on a line which crosses thedirection of the main flow direction of the fluid to be measured.

In the flow rate detecting element constituted as described above, thecomparative resistance section and the heating resistance section aredisposed on the line which crosses the direction of the main flow of thefluid to be measured. Accordingly, the fluid can be prevented frommaking contact with the heating resistance section after making contactwith the comparative resistance section or from making contact with thecomparative resistance section after making contact with the heatingresistance section, thereby preventing the comparative resistancesection and the heating resistance section from giving thermal influenceto each other. As a result, the comparative resistance section of theflow rate detecting element can sense the fluid temperature moreaccurately without being affected by the heating resistance section.Also the heating resistance section can sense the temperature change dueto the interaction with the fluid more accurately without being affectedby the comparative resistance section.

The flow rate detecting element according to the sixth aspect of thepresent application is a variation of the flow rate detecting element ofthe first aspect, wherein the comparative resistance section and theheating resistance section are disposed on a line which crosses thedirection of the main flow of the fluid to be measured at right angles.

In the flow rate detecting element constituted as described above, thecomparative resistance section and the heating resistance section aredisposed on the line which crosses the direction of the main flow of thefluid to be measured at right angles. Accordingly, the fluid can beprevented from making contact with the heating resistance section aftermaking contact with the comparative resistance section or from makingcontact with the comparative resistance section after making contactwith the heating resistance section, thereby preventing the comparativeresistance section and the heating resistance section from givingthermal influence to each other. As a result, the comparative resistancesection of the flow rate detecting element can sense the fluidtemperature more accurately without being affected by the heatingresistance section. Also the heating resistance section can sense thetemperature change due to the interaction with the fluid without beingaffected by the comparative resistance section.

The seventh aspect of the present invention provides a thermosensitiveflow rate detecting element which has a thin film layer comprising asupport film and a protective film, both made of insulating material andformed on one surface of a flat substrate, where a heating resistancesection and a comparative resistance section are provided by disposingthermosensitive resistor in predetermined patterns between the supportfilm and the protective film of the thin film layer, and the flatsubstrate has a recess which penetrates the flat substrate in thedirection of thickness thereof provided in at least portions thereofthat face the heating resistance section and the comparative resistancesection, so that the thermosensitive flow rate detecting elementmeasures the flow rate or velocity of a fluid by means of the heatingresistance section according to the report of fluid temperature sensedby the comparative resistance section, while at least two fluid flowpassages are provided which communicates with the recess that faces thecomparative resistance section to cause the fluid to flow to the recess,with the fluid flow passage being of one type selected from among thegroup consisting of:

(i) a hole which penetrates the thin film layer in the direction ofthickness thereof to flow the fluid across the thin film layer betweenthe upper surface and the lower surface thereof;

(ii) at least one groove which communicate between a recess wall surfacefacing the comparative resistance section and one end wall surface ofthe substrate on the surface of the substrate opposite to the thin filmlayer; and

(iii) at least one tubular passage which communicates between the recesswall surface facing the comparative resistance section and one end wallsurface of the substrate.

In the flow rate detecting element constituted as described above, asthe first feature, the fluid can be caused to flow more smoothly intothe recess by providing at least two fluid flow passages whichcommunicate the recess facing the comparative resistance section to flowthe fluid to the recess, while arranging at least one of the fluid flowpassages at an fluid inlet-side of the recess and at least one of thefluid flow passages at an fluid outlet-side of the recess. Therefore,the comparative resistance section of the flow rate detecting elementcan sense the fluid temperature accurately, and the flow rate detectingelement can make quick response to the temperature change even when thefluid temperature changes suddenly.

In the flow rate detecting element constituted as described above, asthe second feature, a smooth passage can be provided for the fluid toflow through the fluid flow passages to the recess by using at least onetype of flow passage selected from among a group of (i) hole, (ii)groove and (iii) tubular passage as the fluid flow passages.

The flow rate detecting element according to the eighth aspect of thepresent application is a variation of the flow rate detecting element ofthe seventh aspect, wherein the fluid flow passage provided in theupstream is a hole which penetrates the thin film layer in the directionof thickness thereof to let the fluid flow between the upper surface andthe lower surface of the thin film layer, and the fluid flow passageprovided in the downstream is a passage of at least one type selectedfrom among a group consisting of:

(i) a hole which penetrates the thin film layer in the direction ofthickness thereof to flow the fluid across the thin film layer betweenthe upper surface and the lower surface thereof;

(ii) at least one groove which communicates between a recess wallsurface facing the comparative resistance section and one end wallsurface of the substrate on the surface of the substrate opposite to thethin film layer; and

(iii) at least one tubular passage which communicates between a recessedwall surface facing the comparative resistance section and one end wallsurface of the substrate.

In the flow rate detecting element constituted as described above, sincethe hole is used as the fluid flow passage provided in the upstream anda flow passage of at least one type selected from hole, groove andtubular passage is used as the fluid flow passage provided in thedownstream, the fluid can flow through the hole located in the upstreaminto the recess and flow out of the recess through the fluid flowpassage of at least one type selected from hole, groove and tubularpassage, thus securing a smooth passage for the fluid to flow into therecess. As a result, the comparative resistance section has an improvedtemperature response characteristic as the fluid can flow sufficientlyalso to the lower surface of the comparative resistance section.

The flow rate detecting element according to the ninth aspect of thepresent application is a variation of the flow rate detecting element ofthe eighth aspect, wherein a fluid flow passage having a form of slit isprovided in the upstream and the thin film layer is warped, at least ina part of the comparative resistance section, so as to be convex to theopposite side of the substrate.

In the flow rate detecting element constituted as described above, sincethe thin film layer is warped, at least in a part of the comparativeresistance section, so as to be convex to the opposite side of thesubstrate, the fluid can flow more smoothly through the hole into therecess. As a result, the comparative resistance section has an improvedtemperature response characteristic since the fluid can flow moreefficiently to the lower surface of the comparative resistance section.

The flow rate detecting element according to the tenth aspect of thepresent application is a variation of the flow rate detecting element ofthe ninth aspect, wherein a fluid flow passage having a form of slit isprovided in the downstream.

In the flow rate detecting element constituted as described above, sinceboth fluid flow passages disposed in the upstream and the downstream areformed in slits and the thin film layer is warped, at least in a part ofthe comparative resistance section, so as to be convex to the oppositeside of the substrate, the fluid can flow more smoothly through the slitinto the recess. As a result, the comparative resistance section has animproved temperature response characteristic since the fluid can flowmore efficiently to the lower surface of the comparative resistancesection.

The flow rate detecting element according to the eleventh aspect of thepresent application is a variation of the flow rate detecting element ofthe seventh aspect, wherein at least one groove which communicatesbetween the recess wall surface facing the comparative resistancesection and one end wall surface of the substrate is provided on thesurface of the substrate opposite to the thin film surface, as the fluidflow passage in the upstream, and the fluid flow passage provided in thedownstream is a passage of at least one type selected from among a groupconsisting of:

(i) a hole which penetrates the thin film layer in the direction ofthickness thereof to flow the fluid across the thin film layer betweenthe upper surface and the lower surface thereof;

(ii) at least one groove which communicates between a recess wallsurface facing the comparative resistance section and one end wallsurface of the substrate on the surface of the substrate opposite to thethin film layer; and

(iii) at least one tubular passage which communicates between a recesswall surface facing the comparative resistance section and one end wallsurface of the substrate.

In the flow rate detecting element constituted as described above, sincethe groove is used as the fluid flow passage provided in the upstreamand a passage of at least one type selected from hole, groove andtubular passage is used as the fluid flow passage provided in thedownstream, the fluid can flow through the groove located upstream intothe recess and flow out of the recess through the fluid flow passage ofat least one type selected from hole, groove and tubular passage locatedin the downstream, thus securing a smooth passage for the fluid to flowinto the recess. As a result, the comparative resistance section has animproved temperature response characteristic since the fluid can flowsufficiently also to the lower surface of the comparative resistancesection.

The flow rate detecting element according to the twelfth aspect of thepresent invention is a variation of the flow rate detecting element ofthe eleventh aspect, wherein the groove as the fluid flow passage isformed so that the sectional area thereof at the opening in the recesswall surface facing the comparative resistance section or in the endwall surface of the substrate is larger than the sectional area of anyother portion of the groove.

In the flow rate detecting element constituted as described above, sincethe sectional area thereof at the opening in the recess wall surfacefacing the comparative resistance section or in the end wall surface ofthe substrate is larger than the sectional area of any other portion ofthe groove, the fluid is made easier to flow into the groove and flowout of the groove. As a result, the comparative resistance section hasan improved temperature response characteristic since the fluid can flowalso to the lower surface of the comparative resistance section moresmoothly.

The flow rate detecting element according to the thirteenth aspect ofthe present invention is a variation of the flow rate detecting elementof the seventh aspect, wherein at least one tubular passage whichcommunicates between the recess wall surface facing the comparativeresistance section and one end wall surface of the substrate is providedas the fluid flow passage in the upstream, and the fluid flow passageprovided in the downstream is a passage of at least one type selectedfrom among the group consisting of:

(i) a hole which penetrates the thin film layer in the direction ofthickness thereof to flow the fluid across the thin film layer betweenthe upper surface and the lower surface thereof;

(ii) at least one groove which communicates between a recess wallsurface facing the comparative resistance section and one end wallsurface of the substrate on the surface opposite to the thin film layerof the substrate; and

(iii) at least one tubular passage which communicates between a recesswall surface facing the comparative resistance section and one end wallsurface of the substrate.

In the flow rate detecting element constituted as described above, sincethe tubular passage is used as the fluid flow passage provided in theupstream and a passage of at least one type selected from hole, grooveand tubular passage is used as the fluid flow passage provided in thedownstream, the fluid can flow through the tubular passage located inthe upstream into the recess and flow out of the recess through thefluid flow passage of at least one type selected from hole, groove andtubular passage located in the downstream, thus securing a smoothpassage for the fluid to flow into the recess. As a result, thecomparative resistance section has an improved temperature responsecharacteristic since the fluid can flow sufficiently also to the lowersurface of the comparative resistance section.

The invention of element holder according to the first aspect of thepresent application provides an element holder for accommodating athermosensitive flow rate detecting element which comprises a flatsubstrate having a thin film layer that consists of a support film and aprotective film both made of insulating material and laminated on onesurface thereof, wherein a heating resistance section and a comparativeresistance section are provided between the support film and theprotective film of the thin film layer by disposing thermosensitiveresistor in predetermined patterns, the flat substrate has a recesswhich penetrates the flat substrate in the direction of thicknessthereof in at least such portions thereof that face the heatingresistance section and the comparative resistance section, and a fluidflow passage which communicates with the recess facing the comparativeresistance section is provided to flow the fluid to the recess, so thatthe thermosensitive flow rate detecting element measures the flow rateor velocity of the fluid by means of the heating resistance sectionaccording to the report of fluid temperature sensed by the comparativeresistance section, the element holder having airfoil-shaped crosssection with at least one gap opening provided in the holder surface inthe upstream and the downstream portions with respect to the comparativeresistance section.

The element holder constituted as described above causes no significantdisturbance to the fluid flow around the element holder because of theairfoil-shaped cross section, and rather shows flow straightening effectso that the fluid is introduced through the gap opening in the upstreamof the comparative resistance section into the holder and, after makingsufficient contact with the flow rate detecting element accommodatedtherein, particularly with the comparative resistance section, the fluidcan be discharged to the outside of the holder through the gap openinglocated in the downstream of the comparative resistance section. Thusthe fluid can sufficiently make contact with the flow rate detectingelement accommodated therein. As a result, the comparative resistancesection of the flow rate detecting element can exhibit good temperatureresponse characteristic without substantially disturbing the fluid flow.

The invention of element holder according to the second aspect of thepresent application is the element holder of the first aspect, whereinthe gap opening located in the upstream of the comparative resistancesection is provided at a position which corresponds to the upper surfaceof the flow rate detecting element that is accommodated therein.

The element holder constituted as described above, when used inconjunction with the flow rate detecting element which has a holeprovided as the fluid flow passage in the upstream of the flow ratedetecting element, the gap opening located in the upstream of theelement holder and the hole provided as the fluid flow passage of theflow rate detecting element accommodated therein can be disposed so asto oppose each other. As a result, the fluid flow in the element holderand around the flow rate detecting element can be made smoother.

The invention of element holder according to the third aspect of thepresent application is the element holder of the first aspect, whereinthe lower edge of the gap opening located in the upstream of thecomparative resistance section is provided at a position whichcorresponds to the end wall surface of the substrate of the flow ratedetecting element that is accommodated therein.

The element holder constituted as described above, when used inconjunction with the flow rate detecting element which has a tubularpassage that opens in the end wall surface located in the upstream ofthe substrate, the gap opening located in the upstream of the elementholder and the tubular passage provided as the fluid flow passage of theflow rate detecting element accommodated therein can be disposed so asto oppose each other. As a result, the fluid flow in the element holderand around the flow rate detecting element can be made smoother.

The invention of element holder according to the fourth aspect of thepresent application is the element holder of the first aspect, whereinthe lower edge of the gap opening located in the upstream of thecomparative resistance section is provided at a position the same levelas or lower than the lower surface of the substrate or at a positionbelow thereof in the flow rate detecting element that is accommodatedtherein.

The element holder constituted as described above, when used inconjunction with the flow rate detecting element which has a grooveprovided in the lower surface of the substrate as the fluid flow passagelocated in the upstream of the flow rate detecting element, makes itpossible to dispose the gap opening located in the upstream of theelement holder and the groove provided as the fluid flow passage of theflow rate detecting element accommodated therein so as to oppose eachother. As a result, the fluid flow in the element holder and around theflow rate detecting element can be made smoother.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1 b are schematic diagrams showing a basic embodiment ofthe invention related to the flow rate detecting element, particularlyillustrating the features of the first to fourth, ninth and twelfthembodiments, FIG. 1a being a plan view of the comparative resistancesection of the flow rate detecting element and vicinity, and FIG. 1bbeing a sectional view taken along lines Ib—Ib in FIG. 1a.

FIGS. 2a-2 c are schematic diagrams showing a basic embodiment of theinvention related to the flow rate detecting element, particularlyillustrating the features of the thirteenth and fourteenth embodiments,FIG. 2a being a plan view of the comparative resistance section of theflow rate detecting element and vicinity, FIG. 2b being a sectional viewtaken along lines IIb—IIb in FIG. 2a and FIG. 2c being a sectional viewtaken along lines IIc—IIc in FIG. 2a.

FIGS. 3a-3 c are schematic diagrams showing a basic embodiment of theinvention related to the flow rate detecting element, particularlyillustrating the features of the first to fourth and fifteenthembodiments, FIG. 3a being a plan view of the comparative resistancesection of the flow rate detecting element and vicinity, FIG. 3b being asectional view taken along lines IIIb—IIIb in FIG. 3a, and FIG. 3c beingan end view from the right-hand side in FIG. 3a.

FIGS. 4a-4 c are schematic diagrams showing a basic embodiment of theinvention related to the flow rate detecting element, particularlyillustrating the features of the first to fourth, ninth and fifteenthembodiments, FIG. 4a being a plan view of the comparative resistancesection of the flow rate detecting element and vicinity, FIG. 4b being asectional view taken along lines IVb—IVb in FIG. 4a, and FIG. 4c beingan end view from the right-hand side in FIG. 4a.

FIGS. 5a-5 c are schematic diagrams showing a basic embodiment of theinvention related to the flow rate detecting element, particularlyillustrating the features of the first to fourth, ninth and sixteenthembodiments, FIG. 5a being a plan view of the comparative resistancesection of the flow rate detecting element and vicinity, FIG. 5b being asectional view along line Vb—Vb in FIG. 5a, and FIG. 5c being an endview from the right-hand side in FIG. 5a.

FIGS. 6a and 6 b are schematic diagrams showing a basic embodiment ofthe invention related to the flow rate detecting element, particularlyillustrating the features of the fourteenth embodiment, FIG. 6a being aplan view of the comparative resistance section of the flow ratedetecting element and vicinity, FIG. 6b being a sectional view takenalong lines VIb—VIb in FIG. 6a, and FIG. 6c being an end view from theright-hand side in FIG. 6a.

FIGS. 7a-7 c are schematic diagrams showing a basic embodiment of theinvention related to the element holder, particularly illustrating thefeatures of the second and third embodiments of the element holder, FIG.7a being a schematic plan view of the flow rate detecting element andthe element holder which accommodates the flow rate detecting elementbeing cut away by a horizontal surface at the same height as the topsurface of the flow rate detecting element, FIG. 7b being a sectionalview taken along lines VIIb—VIIb in FIG. 7a, and FIG. 7c being asectional view taken along lines VIIa—VIIa in FIG. 7b.

FIGS. 8a-8 c are schematic diagrams showing a basic embodiment of theinvention related to the element holder, particularly illustrating thefeatures of the fourth embodiment of the element holder, FIG. 8a being aschematic plan view of the flow rate detecting element and the elementholder which accommodates the flow rate detecting element being cut awayby a horizontal surface at the same height as the top surface of theflow rate detecting element, FIG. 8b being a sectional view taken alonglines VIIIb—VIIIb in FIG. 8a, and FIG. 8c being an end view in FIG. 8a.FIG. 8a corresponds to the sectional view taken along lines VIIIa—VIIIain FIG. 8b.

FIGS. 9a and 9 b are schematic diagrams showing the basic constitutionof the thermosensitive flow rate detecting element.of diaphragm type ofthe prior art, FIG. 9a schematically showing the flow rate detectingelement accommodated in the element holder, and FIG. 9b being asectional view taken along lines IXb—IXb in FIG. 9a.

FIGS. 10a and 10 b schematically show the process of forming the tubularpassage which penetrates the substrate according to the inventionrelated to the flow rate detecting element, FIG. 10a showing the lowersurface of the substrate before being subjected to electrochemicaletching, FIG. 10b being an end view showing a V groove a formed in thelower surface of the substrate, FIG. 10c being an end view showing agroove b and groove c being formed from the V groove a downward in thelower surface of the substrate, and FIG. 10d being an end view showing atubular passage being formed by filling up the groove b while leavingthe groove c unfilled on the lower surface of the substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventions of the present application will now be described by wayof preferred embodiments.

First Embodiment

The first embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIGS. 1 to6.

In FIG. 1, reference numeral 1 denotes a flat substrate made from, forexample, a silicon wafer about 400 μm thick, with a support film 2 madeof silicon nitride film about 1 μm thick being formed on the uppersurface of the flat substrate 1 by sputtering or other process. Providedon the support film 2 is a thermosensitive resistor made of platinumbeing formed in a predetermined pattern as shown in the drawing with athickness of, for example, 0.2 μm by such process as vapor deposition orsputtering, thereby forming a comparative resistance section 4. Thecomparative resistance section 4 is a current path formed by patterningwith such as photomechanical process, wet etching process or dry etchingprocess. Formed by sputtering or the like on the top surface of thesupport film 2 and the comparative resistance section 4 is a protectivefilm 3 made of silicon nitride film about 1 μm thick. The layercomprising the support film 2 and the protective film 3 which sandwichthe comparative resistance section 4 will be referred to as the thinfilm layer in this specification.

The lower surface of the flat substrate 1, namely the surface oppositeto that whereon the support film 2 is formed, is covered by a backsurface protecting film 5. The back surface protecting film 5 is a layermade of, for example, SiO₂ for the protection of the lower surface ofthe substrate.

Then an etching hole corresponding to the shape of the recess to beformed in the lower surface of the substrate is formed into the lowersurface of the back surface protecting film 5. In the drawing, referencenumeral 17 indicates the outline of the etching hole. The etching holeis formed by photoengraving or other process.

After the etching hole has been formed, the substrate 1 is partiallyremoved by an etching process, for example alkali etching, thereby toform the recess 7, which has a predetermined shape in plan view(substantially rectangular in FIG. 1a) and penetrates the flat substratein the direction of thickness, in the lower surface of the substrate 1.As shown in FIG. 1b, the recess 7 has a longitudinal section ofsubstantially trapezoidal shape, expanding in width toward the lowersurface of the substrate. Dimensions and shape of the recess 7 may bedetermined by a technique known in the prior art or may be modified asrequired by the design.

In FIG. 1a, a portion of the thin film layer which faces the recess 7and includes the comparative resistance section 4 and an areasurrounding thereof is enclosed by two-dot and a dash line in arectangular shape, which will be referred to as the comparativeresistance region 8 for the convenience of description. Since the recess7 is formed on the lower surface of the comparative resistance region 8of the flow rate detecting element, the thin film layer can be said toconstitute a so-called diaphragm structure in the comparative resistanceregion 8.

The flow rate detecting element has a fluid flow passage whichcommunicates with the recess 7 thereby to flow a fluid into the recess7.

According to the invention related to the flow rate detecting element ofthe first embodiment of the present application, the fluid flow passagemay have any size, shape or form as long as the basic function ofcommunicating with the recess 7 and flowing a fluid into the recess 7can be performed.

For particular forms of the fluid flow passage, the form of holes 9which penetrate the thin film layer in the direction of thickness onboth sides of the comparative resistance section as shown in FIGS. 1 and2, the form of grooves 13, 14, 16 formed on the lower surface of thesubstrate as shown in FIGS. 3, 4, 6, and the form of tubular passages 15which penetrates the substrate as shown in FIG. 5 can be shown.

The hole shown in FIG. 1 has a form of narrow rectangle (slit) extendingat right angles to the direction of main flow of the fluid indicated byarrow 10 in the plan view of FIG. 1a and is, as shown in thelongitudinal view of FIG. 1b, defined by two vertical walls which areformed by cutting off the thin film layer vertically so as to besubstantially parallel to each other, although the present invention isnot limited to this configuration. The fluid flow passage may betriangle, rectangle or other polygonal shape, rounded shape such ascircle, elongated circle or oval, or a shape enclosed by a series ofstraight lines and/or curves in the plan view. The outline of the wallsurface of the substrate appearing in the sectional view may also beconstituted from straight lines inclined by various angles or variouscurves, or a combination thereof.

The term “direction of main flow of the fluid” used in the presentinvention means the direction in which the predominant velocitycomponent of the fluid flow lies. Accordingly, the direction of mainflow of the fluid is directed from the left to the right in all examplesshown in FIGS. 1 to 8, even when the fluid involves local disturbance orvortex. The fluid flow is indicated by the arrow 10 in FIGS. 1 to 8.

The fluid flow passage in the form of hole or slit can be easily formedby etching the thin film layer by dry etching or the like, in the stateof the thin film layer having been formed on the substrate.

In FIG. 1, the fluid is assumed to flow from the left to the right asindicated by the arrow 10 in FIG. 1b, and the two fluid flow passagesare provided in the upstream and downstream of the comparativeresistance region 8. However, there are no limitations to the number andpositions of the fluid flow passages to be provided as long as the fluidcan flow smoothly into the recess 7 which is provided in the lowersurface of the thin film layer. Thus a case of providing a single fluidflow passage is also included in the scope of the invention related tothe first embodiment, regardless of whether the fluid flow passage isinstalled over or below the comparative resistance section 4 in FIG. 1a.Direction of the fluid flow during use is also not limited to that shownin the drawing.

The flow rate detecting element constituted as described above makes itpossible to bring the fluid into sufficient contact also with the lowersurface of the comparative resistance region which has the diaphragmstructure, by the fluid flow passage communicating with the recess thatfaces the comparative resistance region and flowing the fluid smoothlythrough the fluid flow passage into the recess. Thus since thecomparative resistance section of the flow rate detecting element canmake contact with the fluid on both the upper surface and the lowersurface thereof and the fluid which contacts the lower surface alsoflows quickly through the fluid flow passage, the fluid temperature canbe sensed sensitively. As a result, the flow rate detecting element canmake quick response to the temperature change even when the fluidtemperature changes suddenly.

Second Embodiment

The second embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIGS. 1 to 6.

The invention related to the flow rate detecting element of the secondembodiment is a variation of the flow rate detecting element of thefirst embodiment, wherein at least two fluid flow passages are provided.

In the example shown in FIGS. 1 and 2, fluid flow passages 9 in the formof slit are provided at two positions on both sides of the comparativeresistance section 4, to the left and right. In the example shown inFIGS. 3 and 6, three grooves 13 are provided on the left side of thecomparative resistance section 4 and three grooves 13 are provided alsoon the right side of the comparative resistance section 4, while in theexample shown in FIG. 4 one wide groove 14 is provided on the left sideof the comparative resistance section 4 and one wide groove 14 isprovided also on the right side of the comparative resistance section 4.In the example shown in FIG. 5, three tubular passages 15 are providedon the left side of the comparative resistance section 4 and threetubular passages 15 are provided also on the right side of thecomparative resistance section 4. Basic constitution of the flow ratedetecting element in any of FIGS. 1 to 6 is the same as that of thefirst embodiment.

In the flow rate detecting element constituted as described above, thefluid can be caused to flow more smoothly into the recess by using atleast one fluid flow passage as an inlet of the fluid and at least onefluid flow passage as an outlet of the fluid. Therefore, the comparativeresistance section of the flow rate detecting element can sense thefluid temperature accurately, and the flow rate detecting element canmake quick response to the temperature change even when the fluidtemperature changes suddenly.

As to the size and shape of the fluid flow passage, many variations maybe employed as in the case of the first embodiment.

Third Embodiment

The third embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIGS. 1 to6.

The invention related to the flow rate detecting element of the thirdembodiment is a variation of the flow rate detecting element of thefirst embodiment, wherein at least one fluid flow passage is provided inthe upstream of the comparative resistance section in the main flowdirection of the fluid to be measured.

In the example shown in FIGS. 1 to 6, main flow direction of the fluidto be measured is from the left to the right as indicated by the arrow10 in the drawing. In all of the cases shown in FIGS. 1 to 6, the fluidflow passage is disposed in the upstream of the comparative resistancesection 4, namely on the right side of the comparative resistancesection 4. Basic constitution of the flow rate detecting element in anyof FIGS. 1 to 6 is the same as that of the first embodiment.

In the flow rate detecting element constituted as described above, it ismade easier for the fluid to flow through the fluid flow passage intothe recess so that the fluid can be caused to flow more smoothly throughthe fluid flow passage into the recess, by providing at least one fluidflow passage in the upstream of the comparative resistance section inthe main flow direction of the fluid to be measured. Therefore, thecomparative resistance section of the flow rate detecting element cansense the fluid temperature accurately, and the flow rate detectingelement can make quick response to the temperature change even when thefluid temperature changes suddenly.

Forth Embodiment

The fourth embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIGS. 1 to 6.

The invention related to the flow rate detecting element of the fourthembodiment is a variation of the flow rate detecting element of thefirst embodiment, wherein at least one fluid flow passage is provided inthe downstream of the comparative resistance section in the main flowdirection of the fluid to be measured.

In the example shown in FIGS. 1 to 6, the main flow direction of thefluid to be measured is from the left to the right as indicated by thearrow 10 in the drawing. Thus in all of the cases shown in FIGS. 1 to 6,the fluid flow passage is provided in the downstream of the comparativeresistance section 4, namely on the right side of the comparativeresistance section 4. Basic constitution of the flow rate detectingelement in any of FIGS. 1 to 6 is the same as that of the firstembodiment.

In the flow rate detecting element constituted as described above, it ismade easier for the fluid to flow through the fluid flow passage intothe recess so that the fluid can be caused to flow more smoothly intothe recess, by providing at least one fluid flow passage in thedownstream of the comparative resistance section in the main flowdirection of the fluid to be measured. Therefore, the comparativeresistance section of the flow rate detecting element can sense thefluid temperature accurately, and the flow rate detecting element canmake quick response to the temperature change even when the fluidtemperature changes suddenly.

Fifth Embodiment

The fifth embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIGS. 7 and8.

The invention related to the flow rate detecting element of the fifthembodiment is a variation of the flow rate detecting element of thefirst embodiment, wherein the comparative resistance section and theheating resistance section are disposed on a line which crosses thedirection of the main flow direction of the fluid to be measured.

In the example shown in FIGS. 7 and 8, the fluid flow passages areprovided on both sides of the comparative resistance section 25, and themain flow direction of the fluid to be measured is from the left to theright as indicated by the arrow 10 in the drawing.

In the flow rate detecting element 23 shown in FIGS. 7 and 8, since thefluid flow passages are provided so as to bring the fluid into goodcontact with the upper and lower surfaces of the comparative resistancesection 25 the heating resistance section cannot be provided in thedownstream of the comparative resistance section as in the case shown inFIG. 9, in order to prevent the heating resistance section from beingthermally influenced by the comparative resistance section. Therefore,when the flow rate detecting element of this invention is used,arrangement of the comparative resistance section and the heatingresistance section should be determined so that the heating resistancesection is free from the thermal influence of the comparative resistancesection or is least likely to be influenced thereby.

For this reason, the comparative resistance section 25 and the heatingresistance section 24 in the example shown in FIG. 7 are disposed on aline which crosses the direction of the main flow of the fluid to bemeasured. The example shown in FIG. 8 is also constituted similarly.

The line which crosses the direction of the main flow of the fluid,mentioned above, may be any line except for the line which is parallelto the main flow direction of the fluid. Therefore, all embodimentsexcept for one wherein the comparative resistance section and theheating resistance section are disposed on a line parallel to thedirection of the main flow of the fluid to be measured are included inthe scope of this invention. Thus the scope of this invention includesnot only such an arrangement where the comparative resistance sectionand the heating resistance section are disposed on a line, which crossesthe direction of the main flow of the fluid to be measured, on the samesurface of one flat substrate, but also such an arrangement where two ormore flat substrates are laminated with proper space therebetween withthe comparative resistance section being provided on the surface of onesubstrate and the heating resistance section being provided on thesurface of a substrate above or below the former.

In the flow rate detecting element constituted as described above, thecomparative resistance section and the heating resistance section aredisposed on the line which crosses the direction of the main flow of thefluid to be measured. Accordingly, the fluid can be prevented frommaking contact with the heating resistance section after making contactwith the comparative resistance section or from making contact with thecomparative resistance section after making contact with the heatingresistance section, thereby preventing the comparative resistancesection and the heating resistance section from giving thermal influenceto each other. As a result, the comparative resistance section of theflow rate detecting element can sense the fluid temperature moreaccurately without being affected by the heating resistance section.Also the heating resistance section can sense the temperature change dueto the interaction with the fluid more accurately without being affectedby the comparative resistance section.

Sixth Embodiment

The sixth embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIGS. 7 and8.

The invention related to the flow rate detecting element of the sixthembodiment is a variation of the flow rate detecting element of thefirst embodiment, wherein the comparative resistance section and theheating resistance section are disposed on a line which perpendicularlycrosses the direction of the main flow of the fluid to be measured.

In the example shown in FIGS. 7 and 8, the fluid flow passages areprovided on both sides of the comparative resistance section 25, to theleft and right, and the main flow direction of the fluid to be measuredis from the left to the right as indicated by the arrow 10 in thedrawing.

In this embodiment, too, the heating resistance section cannot beprovided in the downstream of the comparative resistance section as inthe case shown in FIG. 9, for the same reason as that of the fifthembodiment. Therefore, when the flow rate detecting element of thisinvention is used, arrangement of the comparative resistance section andthe heating resistance section should be determined so that the heatingresistance section is free from the thermal influence of the comparativeresistance section or is least likely to be influenced thereby.

For this reason, the comparative resistance section 25 and the heatingresistance section 24 in the example shown in FIG. 7 are disposed on aline which perpendicularly crosses the direction of the main flow of thefluid to be measured. The example shown in FIG. 8 is also constitutedsimilarly.

In the flow rate detecting element constituted as described above, thecomparative resistance section and the heating resistance section aredisposed on the line which perpendicularly crosses the direction of themain flow of the fluid to be measured. Accordingly, the fluid can beprevented from making contact with the heating resistance section aftermaking contact with the comparative resistance section or from makingcontact with the comparative resistance section after making contactwith the heating resistance section, thereby preventing the comparativeresistance section and the heating resistance section from givingthermal influence to each other. As a result, the comparative resistancesection of the flow rate detecting element can sense the fluidtemperature more accurately without being affected by the heatingresistance section. Also the heating resistance section can sense thetemperature change due to the interaction with the fluid without beingaffected by the comparative resistance section.

Seventh Embodiment

The seventh embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIGS. 1 to 6.

The invention related to the flow rate detecting element of the seventhembodiment has such a constitution as, at least two fluid flow passagesare provided which communicate with the recess that faces thecomparative resistance section so as to cause the fluid to flow to therecess, with the fluid flow passage being of one type selected fromamong the group consisting of:

(i) a hole which penetrates the thin film layer in the direction ofthickness thereof to flow the fluid across the thin film layer betweenthe upper surface and the lower surface thereof;

(ii) at least one groove which communicates between a recess wallsurface facing the comparative resistance section and one end wallsurface of the substrate on the surface of the substrate opposite to thethin film layer; and

(iii) at least one tubular passage which communicates between a recesswall surface facing the comparative resistance section and one end wallsurface of the substrate.

In the example shown in FIGS. 1 and 2, the fluid flow passages in theform of slit holes 9 are provided at two positions on both sides of thecomparative resistance section 4, to the left and right thereof. In theexample shown in FIGS. 3 and 6, the fluid flow passages in the forms ofthree grooves 13, 16 are provided on each side of the comparativeresistance section 4, to the left and right thereof. In the exampleshown in FIG. 4, the fluid flow passage in the form of one wide groove14 is provided on each side of the comparative resistance section 4. Inthe example shown in FIG. 5, the fluid flow passages in the form ofthree tubular passages 15 are provided on each side of the comparativeresistance section 4, to the left and right thereof. Basic constitutionof the flow rate detecting element in any of FIGS. 1 to 6 is the same asthat of the first embodiment.

(i) Although the hole which penetrates the thin film layer in thedirection of thickness and causes the fluid to flow between the uppersurface and the lower surface of the thin film layer is provided in aninner region of the comparative resistance region 8 in the example shownin FIG. 1, the hole may also be provided on the outside of thecomparative resistance region 8, in the portion where the substrate 1exists on the lower surface of the thin film layer, as long as thefunction to flow the fluid between the upper surface and the lowersurface of the thin film layer can be performed.

The shapes of the hole in the plan view and in the longitudinalsectional view thereof are not limited to those shown in the drawing, asmentioned in conjunction with the first embodiment. The fluid flowpassage may be formed in triangle, rectangle or other polygonal shape,rounded shape such as circle, elongated circle or oval, or a shapeenclosed by a series of straight lines and/or curves in the plan view.The outline of the wall surface of the substrate appearing in thesectional view may also be constituted from straight lines inclined byvarious angles or various curves, or a combination thereof.

(ii) At least one groove which communicates between the recess wallsurface that faces the comparative resistance section and one end wallsurface of the substrate on the surface of the substrate opposite to thethin film layer has a substantially triangular cross section at theopening of the groove 13 as shown in FIG. 3c, in the example shown inFIG. 3, the groove 13 which has substantially triangular cross sectionextends so as to communicate between the recess wall surface 6 thatfaces the comparative resistance section and one end wall surface 18 ofthe substrate. In the example shown in FIG. 4, the groove 14 hassubstantially trapezoidal cross section at the opening of the groove asshown in FIG. 4c, so that the groove 14 having substantially trapezoidalcross section extends so as to communicate between the recess wallsurface 6 that faces the comparative resistance section and one end wallsurface 18 of the substrate. In the example shown in FIG. 6, althoughthe overall shape of the groove is similar to that of the example shownin FIG. 3, sectional area of the groove 16 at the opening in the recesswall surface 6 facing the comparative resistance section or in the endwall surface 18 of the substrate is made larger than the sectional areaof any other portion of the groove 16. Thus in the flow rate detectingelement of the seventh embodiment, the fluid can flow smoothly throughthe groove in case the recess wall surface 6 which faces the comparativeresistance section and one end wall surface 18 of the substrate arecommunicated with each other by the groove.

While the groove has the opening formed in triangular or rectangularcross section and has a straight outline extending straight in the planview in the examples shown in these drawings, this invention is notlimited to this configuration and the opening of the groove may havecross section of various shapes similarly to the case of the hole inplan view. The outline of the groove in plan view may also have a shapeconstituted from a combination of one or more straight line and/or curvein the plan view.

(iii) At least one tubular passage which communicates between the recesswall surface that faces the comparative resistance section and one endwall surface of the substrate is illustrated by the example shown inFIG. 5, which means a fluid flow passage 15 that penetrates thesubstrate 1 like a tube, with one end thereof opening in the recess wall6 of the substrate that surrounds the recess and the other end openingin one end wall surface 18 of the substrate 1. While the opening of thetubular passage has rectangular cross section and has a straight outlineextending straight in the plan view in the examples shown in FIG. 5,this invention is not limited to this configuration and the opening ofthe tubular passage may have cross section of various shapes similarlyto the case of the hole in plan view. The outline of the tubular passagein plan view may also have a shape constituted from a combination of oneor more straight line and/or curve in the plan view. Thus in the flowrate detecting element of the seventh embodiment, the fluid can becaused to flow smoothly through the tubular passage, since the tubularpassage communicates between the recess wall surface 6 and the end wallsurface 18 of the substrate.

Such a tubular passage can be formed in a silicon substrate by employinga technique called the electrochemical etching. The technique ofelectrochemical etching is described in detail by H. Ohji, P. T. J.Gennisen, P. J. French and K. Tsutumi in “FABRICATION OF ACCELEROMETERUSING SINGLE-STEP ELECTROCHEMICAL ETCHING FOR MICRO STRUCTURES (SEEM)”(IEEE MEMS Workshop 1999, Orlando, USA, pp61-65), the contents of whichis incorporated by reference herein.

Specific process of forming the tubular passage will be described belowwith reference to FIGS. 10a, b, c, and d. Forming of the thin film layeron the upper surface of the flat substrate 1 and covering of the lowersurface of the substrate 1 with the back surface protecting film 5 arecarried out similarly to the process described in the first embodiment.

FIG. 10a is a bottom view of the substrate 1 shown in FIG. 5a beforebeing subjected to the electrochemical etching, being rotated by 90° tothe left. FIG. 10b to FIG. 10d are end views of the substrate 1 of FIG.10a viewed in the direction of arrow [Xb-Xd]. In other words, FIG. 10bto FIG. 10d show the flat substrate 1 shown in FIG. 5 up side down.Steps of the electrochemical etching process will be described belowwith reference to these drawings.

First, as shown in FIG. 10a, a line 19 which becomes a reference for theplanar configuration of the tubular passage 15 to be formed viewed fromthe back surface protecting film 5 side is drawn on the surface of theback surface protecting film 5 by photoengraving or the like. The planarconfiguration of the tubular passage 15 shown in FIG. 10a has a form ofnarrow rectangle extending between the recess wall surface 6 and the endwall surface 18, and the line 19 which makes reference far theindividual tubular passage 15 is indicated by an alternate dot and dashline. Since three tubular passages 15 are to be formed between one endwall surface 18 and the recess wall surface 6 in FIG. 10a, three lines19 corresponding thereto are formed. The substrate 1 having the lines 19drawn thereon is subjected to alkali etching or the like, thereby toform grooves a having V-shaped longitudinal section at the positions ofthe lines 19 as shown in FIG. 10b. Electrochemical etching by using theV-shaped grooves a as the initial pit results in the formation of grooveb which extends from the V-shaped grooves a vertically downward (in thedirection of thickness of the substrate) as shown in FIG. 10c. When thegrooves b have attained a predetermined depth, widths of the grooves areincreased in the horizontal direction (namely in the direction of thesubstrate surface) by increasing the current density, thereby formingrelatively wide grooves c which has rectangular cross section at thedistal ends of the relatively narrow grooves b that extend vertically asshown in FIG. 10c. Then the grooves b are filled up by CVD process orthe like, so that only the grooves c remain in the substrate 1 as shownin FIG. 10d. The tubular passages 15 which penetrate the substrate 1like tube are formed in the process described above.

In the flow rate detecting element constituted as described above, asthe first feature, the fluid can be caused to flow more smoothly intothe recess by providing at least two fluid flow passages whichcommunicate with the recess wall surface which faces the comparativeresistance section so as to flow the fluid to the recess, whilearranging at least one of the fluid flow passages at an fluid inlet-sideof the recess and at least one of the fluid flow passages at an fluidoutlet-side of the recess. Therefore, the comparative resistance sectionof the flow rate detecting element can sense the fluid temperatureaccurately, and the flow rate detecting element can make quick responseto the temperature change even when the fluid temperature changessuddenly.

In the flow rate detecting element constituted as described above, asthe second feature, a smooth passage can be provided for the fluid toflow through the fluid flow passages to the recess by using at least onetype of passage selected from among a group consisting of (i) hole, (ii)groove and (iii) tubular passage as the fluid flow passages.

While it is preferable to employ the fluid flow passages of at least onetype selected from among a group consisting of (i) hole, (ii) groove and(iii) tubular passage for the fluid flow passages to be formed in onesubstrate from the view point of the production efficiency and costs, acombination of (i) hole and/or (ii) groove and/or (iii) tubular passagemay be employed as required. Specifically, such a constitution may beemployed as a fluid flow passage comprising a combination of (i) holeand/or (ii) groove and/or (iii) tubular passage is provided in theupstream of the comparative resistance section 4 and a fluid flowpassage comprising a combination of (i) hole and/or (ii) groove and/or(iii) tubular passage is provided in the downstream of the comparativeresistance section 4, with the combination located in the downstreambeing either the same as or different from the combination located inthe upstream.

Eighth Embodiment

The eighth embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIGS. 1 and 2.

The invention related to the flow rate detecting element of the eighthembodiment is the flow rate detecting element of the seventh embodiment,wherein a flow passage having the form of (i) hole is provided in theupstream of the comparative resistance section 4 and a flow passage ofat least one type selected from among a group consisting of (i) hole,(ii) groove and (iii) tubular passage is provided in the downstream ofthe comparative resistance section 4. Basic constitutions of the flowrate detecting element shown in FIGS. 1 to 6 are the same as those ofthe first to seventh embodiments. The forms of (i) hole, (ii) groove and(iii) tubular passage are also the same as those described in theseventh embodiment.

In the flow rate detecting element constituted as described above, sincethe hole is used as the fluid flow passage provided in the upstream anda passage of at least one type selected from hole, groove and tubularpassage is used as the fluid flow passage provided in the downstream,the fluid can flow through the hole 9 located in the upstream into therecess 7 and flow out of the recess through the fluid flow passage of atleast one type selected from hole, groove and tubular passage, thussecuring a smooth passage for the fluid to flow into the recess 7. As aresult, the comparative resistance section has an improved temperatureresponse characteristic since the fluid can flow also to the lowersurface of the comparative resistance section.

Ninth embodiment

The ninth embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIG. 2.

The invention related to the flow rate detecting element of the ninthembodiment is the flow rate detecting element of the eighth embodiment,wherein a fluid flow passage 9 having a form of slit is provided in theupstream and the thin film layer is warped, at least in a part of thecomparative resistance section, so as to be convex to the opposite sideof the substrate.

The thin film layer can be formed into the shape shown in FIG. 2 bydepositing the thin film layer on the upper surface side of the flatsubstrate 1 in such a way as tensile stress is generated in the supportfilm 2 and compressive stress in the protective film 11. For thispurpose, for example, silicon nitride can be used as the material of thesupport film 2 and silicon oxide can be used as the material of theprotective film 11. Moreover, since the thin film layer has the hole 9formed in slit shape as the fluid flow passage in the comparativeresistance region 8 located in the upstream of the comparativeresistance section 4, middle portion 12 of the diaphragm is warped tobecome convex on the upper side (fluid side) when formed into diaphragmstructure due to the stress generated in the film.

In the flow rate detecting element constituted as described above, sincethe thin film layer is warped, at least in a part of the comparativeresistance section, so as to be convex to the opposite side of thesubstrate, the fluid can flow more smoothly through the hole 9 of slitshape into the recess 7. As a result, the comparative resistance sectionhas an improved temperature response characteristic since the fluid canflow more efficiently to the lower surface of the comparative resistancesection.

Tenth Embodiment

The tenth embodiment of the invention related to the flow rate detectingelement of the present application is schematically shown in FIG. 2.

The invention related to the flow rate detecting element of the tenthembodiment is the flow rate detecting element of the eighth embodiment,wherein a fluid flow passage having a form of slit is provided in thedownstream.

Means for warping the thin film layer to become convex on the upper side(fluid side) is similar to that described in the ninth embodiment.

In the example shown in FIG. 2, the hole 9 having the form of slit isprovided as the fluid flow passage in the comparative resistance region8 in the downstream of the comparative resistance section 4.

In the flow rate detecting element constituted as described above, sinceboth fluid flow passages in the upstream and the downstream are formedas the hole 9 in slit shape and the thin film layer is warped, at leastin a part of the comparative resistance section, so as to be convex tothe opposite side of the substrate, the fluid can flow more smoothlythrough the hole 9 into the recess 7. As a result, the comparativeresistance section has an improved temperature response characteristicsince the fluid can flow more efficiently to the lower surface of thecomparative resistance section.

Eleventh Embodiment

The eleventh embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIGS. 3, 4 and 6.

The invention related to the flow rate detecting element of the eleventhembodiment is the flow rate detecting element of the seventh embodiment,wherein a flow passage having the form of (ii) groove is provided in theupstream of the comparative resistance section 4 and a flow passage ofat least one type selected from among a group consisting of (i) hole,(ii) groove and (iii) tubular passage is provided in the downstream ofthe comparative resistance section 4. Basic constitutions of the flowrate detecting element shown in FIGS. 1 to 6 are the same as those ofthe first and seventh embodiments. The forms of (i) hole, (ii) grooveand (iii) tubular passage are also the same as those described in theseventh embodiment.

In the flow rate detecting element constituted as described above, sincethe groove is used as the fluid flow passage provided in the upstreamand a passage of at least one type selected from hole, groove andtubular passage is used as the fluid flow passage provided in thedownstream, the fluid can flow through the groove located in theupstream into the recess 7 and flow out of the recess 7 through thefluid flow passage of at least one type selected from hole, groove andtubular passage, thus securing a smooth passage for the fluid to flowinto the recess. As a result, the comparative resistance section has animproved temperature response characteristic since the fluid can flowsufficiently also to the lower surface of the comparative resistancesection.

Twelfth embodiment

The twelfth embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIG. 6.

The flow rate detecting element according to the twelfth embodiment ofthe present invention is a variation of the flow rate detecting elementof the eleventh embodiment, wherein the groove 16 used as the fluid flowpassage is formed so that the sectional area thereof at the opening inthe recess wall surface 6 facing the comparative resistance section orthe end wall surface 18 of the substrate is larger than the sectionalarea of any other portion of the groove. Basic constitution of the flowrate detecting element is the same as those of the first and seventhembodiments. The forms of (i) hole, (ii) groove and (iii) tubularpassage are also the same as those described in the seventh embodiment.

In the flow rate detecting element constituted as described above, sincethe sectional area of the groove 16 at the opening in the recess wallsurface 6 facing the comparative resistance section or in the end wallsurface 18 of the substrate is larger than the sectional area of anyother portion of the groove, the fluid is made easier to flow into thegroove and flow out of the groove. As a result, the comparativeresistance section has an improved temperature response characteristicsince the fluid can flow also to the lower surface of the comparativeresistance section more smoothly.

Thirteenth Embodiment

The thirteenth embodiment of the invention related to the flow ratedetecting element of the present application is schematically shown inFIG. 5.

The invention related to the flow rate detecting element of thethirteenth embodiment is the flow rate detecting element of the seventhembodiment, wherein a flow passage having the form of (iii) tubularpassage 15 is provided in the upstream of the comparative resistancesection 4 and a flow passage of at least one type selected from among agroup consisting of (i) hole, (ii) groove and (iii) tubular passage isprovided in the downstream of the comparative resistance section 4.Basic constitutions of the flow rate detecting element shown in FIGS. 1to 6 are the same as those of the first and seventh embodiments. Theforms of (i) hole, (ii) groove and (iii) tubular passage are also thesame as those described in the seventh embodiment.

In the flow rate detecting element constituted as described above, sincethe tubular passage 15 is used as the fluid flow passage provided in theupstream and a passage of at least one type selected from hole, grooveand tubular passage is used as the fluid flow passage provided in thedownstream, the fluid can flow through the tubular passage located inthe upstream into the recess and flow out of the recess 7 through thefluid flow passage of at least one type selected from hole, groove andtubular passage, thus securing a smooth passage for the fluid to flowinto the recess 7. As a result, the comparative resistance section hasan improved temperature response characteristic since the fluid can flowsufficiently also to the lower surface of the comparative resistancesection.

Fourteenth Embodiment

The first embodiment of the invention of element holder for the flowrate detecting element of the present application will be describedbelow with reference to FIGS. 7 and 8.

The invention related to the element holder according to the firstembodiment comprises a flat substrate having a thin film layerconsisting of a support film and a protective film both made ofinsulating material and laminated on one surface thereof, wherein aheating resistance section and a comparative resistance section areprovided between the support film and the protective film of the thinfilm layer by forming the thermosensitive resistor in a predeterminedpattern, the flat substrate has a recess which penetrates the flatsubstrate in the direction of thickness thereof in at least suchportions thereof that face the heating resistance section and thecomparative resistance section, and a fluid flow passage whichcommunicates with the recess wall surface facing the comparativeresistance section is provided to flow the fluid to the recess, so thatthe thermosensitive flow rate detecting element measures the flow rateor velocity of the fluid by means of the heating resistance sectionaccording to the report of fluid temperature sensed by the comparativeresistance section, while the element holder has airfoil-shaped crosssection with at least one gap opening provided in the holder surface inthe upstream and the downstream portions with respect to the comparativeresistance section.

The thermosensitive flow rate detecting element accommodated by theelement holder of this invention may be any of the thermosensitive flowrate detecting elements which are generally used. Accordingly, thethermosensitive flow rate detecting element of the prior art describedin conjunction with the related art is also included in thethermosensitive flow rate detecting element of the present application.

In FIG. 7, reference numeral 24 denotes a heating resistance section, 25denotes a comparative resistance section, and 23 denotes thethermosensitive flow rate detecting element. This drawing shows only twoelements of the heating resistance section and the comparativeresistance section of the flow rate detecting element, for theconvenience of explanation of the form of accommodating the flow ratedetecting element in the element holder, although those skilled in theart will readily understand that the flow rate detecting element can beprovided with other element as required.

The applicant of this patent application has already disclosed anapplication for patent of the element holder which accommodates the flowrate detecting element in Japanese Patent Kokai Publication No.10-293052. As described in the specification, in case thethermosensitive flow rate detecting element is used as the air intakesensor of an automobile engine, for example, the thermosensitive flowrate sensor provided with the thermosensitive flow rate detectingelement is accommodated in the element holder and is disposed in thedownstream of an air cleaner element. Therefore, the flow rate of airintake can be determined as the intake air makes contact with thethermosensitive flow rate sensor accommodated in the element holderafter passing through the air cleaner element. The element holder has aneffect of straightening the fluid flow around the surface of the elementholder, thereby improving the measurement sensitivity and SIN ratio, andalso has an effect of reducing the change in the flow rate sensingcharacteristic due to deviation in the flow through the fluid flowpassage.

The notion that the element holder of this invention has a form ofairfoil as a whole means that the element holder has an effect ofstraightening the fluid flow around the surface of the element holdersimilarly to the invention disclosed in Japanese Patent KokaiPublication No. 10-293052. The term airfoil refers to the form of wingor fin known in the field of hydrodynamics, and may be any shape whichhas the effect of straightening the flow of fluid around thereof.

The element holder also has at least one gap opening provided in theholder surface in the upstream and the downstream portions with respectto the comparative resistance section of the flow rate detecting elementaccommodated therein as shown in FIGS. 7a and 8 a. The term“accommodate” means that the flow rate detecting element is installed inthe element holder and is used in the state of being integratedtherewith, while the flow rate detecting element may be contained withinthe element holder without any part of the flow rate detecting elementbeing exposed to the outside, or may be contained within the elementholder with at least a part of the flow rate detecting element beingexposed to the outside.

The gap opening provided in the element holder may be located at aposition which corresponds to the upper surface of the flow ratedetecting element accommodated therein, or located at a position whichcorresponds to the end wall surface of the substrate of the flow ratedetecting element accommodated therein, or located in the lower surfaceof the substrate or at a lower position of the flow rate detectingelement accommodated therein. Moreover, position of the opening providedin the holder surface in the upstream with respect to the comparativeresistance section in one element holder and the position of the gapopening located in the downstream may or may not be the same.

According to the invention related to the element holder of thisapplication, since it is aimed at to measure the fluid temperature asaccurately as possible while keeping the fluid in good contact with theflow rate detecting element accommodated therein and, based on thismeasurement, to measure the flow rate of the fluid as accurately aspossible, any configuration which has the gap openings provided so as toachieve this function is considered to be included in the scope of thisinvention.

When the element holder of this invention accommodates the flow ratedetecting element described in this application:

(a) in case the flow rate detecting element has the fluid flow passagein the form of (i) hole which penetrates the thin film layer in thedirection of thickness thereof and allows the fluid to flow between theupper surface and the lower surface of the thin film layer, then theelement holder preferably has a matching gap opening at a position whichcorresponds to the upper surface of the flow rate detecting elementaccommodated therein;

(b) in case the flow rate detecting element has the fluid flow passagein the form of (ii) at least one groove which communicates between therecess wall surface that faces the comparative resistance section andone end wall surface of the substrate on the surface of the substrateopposite to the thin film layer, then the element holder preferably hasa matching gap opening at a position which corresponds to the end wallsurface of the substrate of the flow rate detecting element accommodatedtherein; and

(c) in case the flow rate detecting element has the fluid flow passagein the form of (iii) at least one tubular passage which communicatesbetween the recess wall surface that faces the comparative resistancesection and one end wall surface of the substrate, then the elementholder preferably has a matching gap opening in the lower surface of thesubstrate or at a lower position of the flow rate detecting elementaccommodated therein. This relationship may be applied similarly to theupstream and downstream of the comparative resistance section.Furthermore, the gap opening may be provided in the element holder at aposition which corresponds to the fluid flow passage, also in case theflow rate detecting element has the fluid flow passage comprising acombination of (i) hole and/or (ii) groove and/or (iii) tubular passagein the upstream and/or the downstream of the comparative resistancesection.

Location of the gap opening provided in the element holder is such aposition as the fluid can be caused to flow only to the comparativeresistance section 25 of the flow rate detecting element as shown inFIGS. 7a and 8 a.

In the form shown in FIG. 7b, the gap openings 22 of the element holder21 are provided at a position corresponding to the upper surface of theflow rate detecting element accommodated therein and at a positioncorresponding to the end wall surface of the substrate of the flow ratedetecting element, in both the upstream and downstream of thecomparative resistance section 25. As will be understood from FIG. 7c,the element holder 21 has the gap opening 22 which opens in the uppersurface. As a result, the element holder 21 straightens the fluid flowand keeps the fluid in good contact with the flow rate detecting elementin the gap opening 22.

In the form shown in FIG. 8b, the gap openings 26 of the element holder21 are provided in the lower surface of the substrate of the flow ratedetecting element accommodated therein or at a lower position, in boththe upstream and downstream of the comparative resistance section 25. Aswill be understood from FIG. 8c, the element holder 21 has the gapopening 26 which opens in one end wall surface. As a result, the elementholder 21 can straighten the fluid flow and keep the fluid in goodcontact with the flow rate detecting element in the gap opening 26.

Fifteenth Embodiment

The second embodiment of the invention of element holder for the flowrate detecting element of the present application will be describedbelow with reference to FIG. 7.

The invention related to the element holder according to this embodimentis a variation of the element holder described in the fourteenthembodiment wherein the lower edge of the gap opening located in theupstream of the comparative resistance section is provided at a positionwhich corresponds to the upper surface of the flow rate detectingelement that is accommodated therein.

FIG. 7 is a schematic diagram showing the basic embodiment of theinvention related to the element holder, in which FIG. 7a being aschematic plan view of the flow rate detecting element and the elementholder 21 which accommodates the flow rate detecting element being cutaway by a horizontal surface at the same height as the top surface ofthe flow rate detecting element, and FIG. 7b being a sectional viewalong line VIIb—VIIb in FIG. 7a.

FIG. 7a corresponds to the sectional view taken along lines VIIa—VIIa inFIG. 7b, showing the flow rate detecting element 23 enclosed by theelement holder 21. FIG. 7c is an end view corresponding to FIG. 7a.

According to this invention, in case (a) wherein the flow rate detectingelement has the fluid flow passage (not shown) in the form of (i) hole,which penetrates the thin film layer in the direction of thicknessthereof and allows the fluid to flow between the upper surface and thelower surface of the thin film layer, provided in the upstream of thecomparative resistance section 25 as the reference, the fluid can besmoothly directed to the surface of the flow rate detecting element byproviding the gap opening 22 to the element holder 21 at a positioncorresponding to the upper surface of the flow rate detecting elementaccommodated therein, in correspondence to the fluid flow passage.

Sixteenth Embodiment

The third embodiment of the invention of element holder for the flowrate detecting element of the present application will be describedbelow with reference to FIG. 7.

The invention related to the element holder according to this embodimentis a variation of the element holder described in the fourteenthembodiment wherein the lower edge of the gap opening 22 located in theupstream of the comparative resistance section 25 is provided at aposition which corresponds to the end wall surface of the substrate ofthe flow rate detecting element that is accommodated therein.

According to this invention, in case (b) wherein the flow rate detectingelement has the fluid flow passage (not shown) in the form of (ii) atleast one groove, which communicates between the recess wall surfacethat faces the comparative resistance section and one end wall surfaceof the substrate on the surface of the substrate opposite to the thinfilm layer, provided in the upstream of the comparative resistancesection 25 as the reference, then the fluid can be smoothly directed tothe surface of the flow rate detecting element by providing the gapopening 22 to the element holder 21 at a position corresponding to theend wall surface of the substrate of the flow rate detecting elementaccommodated therein, in correspondence to the fluid flow passage.

Seventeenth Embodiment

The fourth embodiment of the invention of element holder for the flowrate detecting element of the present application will be describedbelow with reference to FIG. 8.

The invention related to the element holder according to this embodimentis a variation of the element holder described in the fourteenthembodiment wherein the lower edge of the gap opening located in theupstream of the comparative resistance section is provided at a positionwhich corresponds to the upper surface of the flow rate detectingelement that is accommodated therein.

FIG. 8 is a schematic diagram showing the element holder according tothis embodiment, in which FIG. 8a being a schematic plan view of theflow rate detecting element and the element holder which accommodatesthe flow rate detecting element being cut away by a horizontal surfaceat the same height as the top surface of the flow rate detectingelement, FIG. 8b being a sectional view taken along lines VIIIb—VIIIb inFIG. 8a, and FIG. 8c being an end view corresponding to FIG. 8a. FIG. 8acorresponds to the sectional view taken along lines VIIIa—VIIIa in FIG.8b.

According to this invention, in case (c) wherein the flow rate detectingelement has the fluid flow passage (not shown) in the form of (iii) atleast one tubular passage, which communicates between the recess wallsurface that faces the comparative resistance section 25 and one endwall surface of the substrate, being provided in the upstream of thecomparative resistance section 25 as the reference, the fluid can besmoothly directed to the surface of the flow rate detecting element byproviding the gap opening 26 to the element holder 21 in the lowersurface of the substrate of the flow rate detecting element accommodatedtherein or at a lower position, in correspondence to the fluid flowpassage.

EXAMPLES

The temperature response characteristic of the flow rate detectingelement and the element holder of the present application to changes inthe fluid temperature was measured as follows.

In a flow rate measuring chamber such as a wind tunnel wherein a gasflow can be formed with a preset flow rate, the flow rate detectingelement connected to a necessary electric circuit is installed and airof relatively low temperature is caused to flow therethrough with apreset flow rate. Then the air introduced into the chamber is switchedto air heated to a predetermined temperature, and the change inresistance obtained from the flow rate detecting element is measured todetermine the period of time required for the resistance to reach aconstant value. This period is compared with the time obtained by asimilar experiment conducted on the thermosensitive flow rate detectingelement of the prior art used as a reference. Reduction in the time isgiven in terms of percentage improvement in the temperature responsecharacteristic to the fluid temperature change.

Example 1

The flow rate detecting element and the element holder of the embodimentshown in FIG. 8 were fabricated. As for the dimensions of the openingprovided in the element holder, dimensions of the opening 26 shown inFIG. 8c were set to W=2 mm and D=1 mm. The temperature responsecharacteristic to the fluid temperature change was measured using thiselement holder. Improvement in the temperature response characteristicby 30% was obtained in comparison to the case of using the flow ratedetecting element and the element holder of the prior art.

The flow rate detecting element of the present invention has a bettertemperature response characteristic for making quick response to thechange in the fluid temperature to be measured, as described above, andtherefore can be used for measuring the flow rate of various fluids suchas gases and/or liquids, preferably gases, and particularly air and/or amixture of gases, to measure the flow rate of the fluid more accurately.Thus the flow rate detecting element of the present invention can beused preferably for measuring the flow rate of, in particular, the airintake into an internal combustion engine which requires accuratemeasurement at all times.

Also the element holder of the present invention can be preferably usedfor the flow rate sensor of an internal combustion engine in particular,since the element holder straightens the fluid flow while accommodatingthe flow rate detecting element and keeps the fluid in good contactbetween the flow rate detecting element and the fluid.

What is claimed is:
 1. A thermosensitive flow rate detecting elementcomprising: a flat substrate having a surface and end wall transverse tothe surface; a thin film layer comprising an insulating support film andan insulating protective film on the surface of the flat substrate; aheating resistance section including a thermally sensitive resistorhaving a pattern and a comparative resistance section including athermally sensitive resistor having a pattern, the heating resistancesection and the comparative resistance section being disposed betweenthe support film and the protective film of the thin film layer; arecess which penetrates the flat substrate in a thickness direction andfaces the heating resistance section and the comparative resistancesection; and at least one fluid flow passage through which the recess isin fluid communication with one of a surface of the thin film layerfacing away from the recess and one of the end wall surfaces of thesubstrate, wherein the thermosensitive flow rate detecting elementmeasures one of flow rate and velocity of the fluid, using the heatingresistance section, according to fluid temperature sensed by thecomparative resistance section.
 2. The flow rate detecting elementaccording to claim 1, having at least two fluid flow passages.
 3. Theflow rate detecting element according to claim 1, wherein at least onefluid flow passage is provided upstream of the comparative resistancesection relative to flow direction of the fluid.
 4. The flow ratedetecting element according to claim 1, wherein at least one fluid flowpassage is provided downstream of the comparative resistance sectionrelative to flow direction of the fluid.
 5. The flow rate detectingelement according to claim 1, wherein the comparative resistance sectionand the heating resistance section are disposed on a line crossing flowdirection of the fluid.
 6. The flow rate detecting element according toclaim 1, wherein the comparative resistance section and the heatingresistance section are disposed on a line perpendicular to flowdirection of the fluid.
 7. A thermosensitive flow rate detecting elementcomprising: a flat substrate having opposed first and second surfacesand end wall surfaces transverse to the first and second surfaces; athin film layer comprising an insulating support film and an insulatingprotective film on the first surface of the flat substrate; a heatingresistance section including a thermally sensitive resistor having apattern and a comparative resistance section including a thermallysensitive resistor having a pattern, the heating resistance section andthe comparative resistance section being disposed between the supportfilm and the protective film of the thin film layer; a recess whichpenetrates the flat substrate in a thickness direction and faces theheating resistance section and the comparative resistance section; andat least two fluid flow passages through which the recess is in fluidcommunication with one of a surface of the thin film layer facing awayfrom the recess and one of the end wall surfaces of the substrate, afirst of the two fluid flow passages being upstream of the comparativeresistance section and a second of the fluid flow passages beingdownstream of the comparative resistance section, wherein thethermosensitive flow rate detecting element measures one of flow rateand velocity of the fluid, using the heating resistance section,according to fluid temperature sensed by the comparative resistancesection, and the first of the fluid flow passages is selected from thegroup consisting of (i) a first hole which penetrates the thin filmlayer in a thickness direction so the fluid flows across the thin filmlayer on upper and lower surfaces of the thin film layer, (ii) at leastone groove in the flat substrate and which communicates between a recesswall surface that faces the comparative resistance section and one ofthe end wall surfaces of the substrates, on the second surface of theflat substrate, and (iii) at least one tubular passage within the flatsubstrate and which communicates between the recess wall surface facingthe comparative resistance section and one of the end wall surfaces ofthe substrate.
 8. The flow rate detecting element according to claim 7,wherein the first of the fluid flow passages is the first hole and thesecond of the fluid flow passages is selected from the group consistingof (i) a second hole which penetrates the thin film layer in thethickness direction so the fluid flows across the thin film layerbetween the upper surface and the lower surface, (ii) at least onegroove in the flat substrate and which communicates between the recesswall surface that faces the comparative resistance section and one ofthe end wall surfaces of the substrate, on the second surface of thesubstrate, and (iii) at least one tubular passage within the flatsubstrate and which communicates between the recess wall surface thatfaces the comparative resistance section and one of the end wallsurfaces of the substrate.
 9. The flow rate detecting element accordingto claim 8, wherein the second of the fluid flow passages is the secondhole and each hole of the first and second fluid flow passages is a slitin the thin film layer extending transverse to fluid flow direction. 10.The flow rate detecting element according to claim 9, wherein the thinfilm layer is warped, at least in a part of the comparative resistancesection, and is convex relative to the second side of the substrate. 11.The flow rate detecting element according to claim 7, wherein the firstof the fluid flow passages includes the at least one groove, and thesecond of the fluid flow passages is selected from the group consistingof; (i) a second hole which penetrates the thin film layer in thethickness direction so the fluid flows across the thin film layer on theupper surface and the lower surface, (ii) at least one groove in theflat substrate and which communicates between the recess wall surfacefacing the comparative resistance section and one of the end wallsurfaces of the substrate, on the second surface of the substrate, and(iii) at least one tubular passage within the flat substrate and whichcommunicates between the recess wall surface that faces the comparativeresistance section and one of the end wall surfaces of the substrate.12. The flow rate detecting element according to claim 11, wherein thegroove of the first fluid flow passage has a sectional area at anopening larger than sectional areas of any other portion of the groove.13. The flow rate detecting element according to claim 7, wherein thefirst of the fluid flow passages includes the at least one tubularpassage, and the second of the fluid flow passages is selected from thegroup consisting of (i) a second hole which penetrates the thin filmlayer in the thickness direction so the fluid flows across the thin filmlayer on the upper surface and the lower surface, (ii) at least onegroove in the flat substrate and which communicates between the recesswall surface that faces the comparative resistance section and one ofthe end wall surfaces of the substrate, on the second surface of thesubstrate, and (iii) at least one tubular passage within the flatsubstrate and which communicates between the recess wall surface thatfaces the comparative resistance section and one of the end wallsurfaces of the substrate.
 14. An assembly including an element holderaccommodating a thermosensitive flow rate detecting element, wherein thethermosensitive flow rate detecting element comprises: a flat substrateincluding a surface and end walls and having a thin film layercomprising an insulating support film and an insulating protective filmlaminated on the surface of the flat substrate; a heating resistancesection including a thermally sensitive resistor having a pattern and acomparative resistance section including a thermally sensitive resistorhaving a pattern, the heating resistance section and the comparativeresistance section being disposed between the support film and theprotective film of the thin film layer; a recess which penetrates theflat substrate in a thickness direction and faces the comparativeresistance section, and at least two fluid flow passages through whichthe recess is in fluid communication with one of a surface of the thinfilm layer facing away from the recess and one of the end wall surfacesof the substrate, wherein the flow rate detecting element measures oneof flow rate and velocity of the fluid, using the heating resistancesection, according to fluid temperature sensed by the comparativeresistance section; and the element holder has a cross section with anairfoil shape and at least one gap opening in each of upstream anddownstream portions with respect to the comparative resistance section.15. The assembly according to claim 14, wherein a lower edge of the gapopening located upstream of the comparative resistance section islocated at a position which corresponds to an upper surface of the flowrate detecting element accommodated in the holder.
 16. The assemblyaccording to claim 14, wherein a lower edge of the gap opening locatedupstream of the comparative resistance section is located at a positionwhich corresponds to an end wall surface of the substrate of the flowrate detecting element that is accommodated in the holder.
 17. Theassembly according to claim 14, wherein a lower edge of the gap openinglocated upstream of the comparative resistance section is located at orbelow a lower surface of the substrate of the flow rate detectingelement that is accommodated in the holder.